man/bootup.xml

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   5 <!--
   6   SPDX-License-Identifier: LGPL-2.1+
   7
   8   Copyright 2012 Lennart Poettering
   9 -->
  10
  11 <refentry id="bootup">
  12
  13   <refentryinfo>
  14     <title>bootup</title>
  15     <productname>systemd</productname>
  16
  17     <authorgroup>
  18       <author>
  19         <contrib>Developer</contrib>
  20         <firstname>Lennart</firstname>
  21         <surname>Poettering</surname>
  22         <email>lennart@poettering.net</email>
  23       </author>
  24     </authorgroup>
  25   </refentryinfo>
  26
  27   <refmeta>
  28     <refentrytitle>bootup</refentrytitle>
  29     <manvolnum>7</manvolnum>
  30   </refmeta>
  31
  32   <refnamediv>
  33     <refname>bootup</refname>
  34     <refpurpose>System bootup process</refpurpose>
  35   </refnamediv>
  36
  37   <refsect1>
  38     <title>Description</title>
  39
  40     <para>A number of different components are involved in the system
  41     boot. Immediately after power-up, the system BIOS will do minimal
  42     hardware initialization, and hand control over to a boot loader
  43     stored on a persistent storage device. This boot loader will then
  44     invoke an OS kernel from disk (or the network). In the Linux case,
  45     this kernel (optionally) extracts and executes an initial RAM disk
  46     image (initrd), such as generated by
  47     <citerefentry project='die-net'><refentrytitle>dracut</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
  48     which looks for the root file system (possibly using
  49     <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>
  50     for this). After the root file system is found and mounted, the
  51     initrd hands over control to the host's system manager (such as
  52     <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>)
  53     stored on the OS image, which is then responsible for probing all
  54     remaining hardware, mounting all necessary file systems and
  55     spawning all configured services.</para>
  56
  57     <para>On shutdown, the system manager stops all services, unmounts
  58     all file systems (detaching the storage technologies backing
  59     them), and then (optionally) jumps back into the initrd code which
  60     unmounts/detaches the root file system and the storage it resides
  61     on. As a last step, the system is powered down.</para>
  62
  63     <para>Additional information about the system boot process may be
  64     found in
  65     <citerefentry project='man-pages'><refentrytitle>boot</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para>
  66   </refsect1>
  67
  68   <refsect1>
  69     <title>System Manager Bootup</title>
  70
  71     <para>At boot, the system manager on the OS image is responsible
  72     for initializing the required file systems, services and drivers
  73     that are necessary for operation of the system. On
  74     <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>
  75     systems, this process is split up in various discrete steps which
  76     are exposed as target units. (See
  77     <citerefentry><refentrytitle>systemd.target</refentrytitle><manvolnum>5</manvolnum></citerefentry>
  78     for detailed information about target units.) The boot-up process
  79     is highly parallelized so that the order in which specific target
  80     units are reached is not deterministic, but still adheres to a
  81     limited amount of ordering structure.</para>
  82
  83     <para>When systemd starts up the system, it will activate all
  84     units that are dependencies of <filename>default.target</filename>
  85     (as well as recursively all dependencies of these dependencies).
  86     Usually, <filename>default.target</filename> is simply an alias of
  87     <filename>graphical.target</filename> or
  88     <filename>multi-user.target</filename>, depending on whether the
  89     system is configured for a graphical UI or only for a text
  90     console. To enforce minimal ordering between the units pulled in,
  91     a number of well-known target units are available, as listed on
  92     <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para>
  93
  94     <para>The following chart is a structural overview of these
  95     well-known units and their position in the boot-up logic. The
  96     arrows describe which units are pulled in and ordered before which
  97     other units. Units near the top are started before units nearer to
  98     the bottom of the chart.</para>
  99
 100     <!-- note: do not use unicode ellipsis here, because docbook will replace that
 101          with three dots anyway, messing up alignment -->
 102 <programlisting>local-fs-pre.target
 103          |
 104          v
 105 (various mounts and   (various swap   (various cryptsetup
 106  fsck services...)     devices...)        devices...)       (various low-level   (various low-level
 107          |                  |                  |             services: udevd,     API VFS mounts:
 108          v                  v                  v             tmpfiles, random     mqueue, configfs,
 109   local-fs.target      swap.target     cryptsetup.target    seed, sysctl, ...)      debugfs, ...)
 110          |                  |                  |                    |                    |
 111          \__________________|_________________ | ___________________|____________________/
 112                                               \|/
 113                                                v
 114                                         sysinit.target
 115                                                |
 116           ____________________________________/|\________________________________________
 117          /                  |                  |                    |                    \
 118          |                  |                  |                    |                    |
 119          v                  v                  |                    v                    v
 120      (various           (various               |                (various          rescue.service
 121     timers...)          paths...)              |               sockets...)               |
 122          |                  |                  |                    |                    v
 123          v                  v                  |                    v              <emphasis>rescue.target</emphasis>
 124    timers.target      paths.target             |             sockets.target
 125          |                  |                  |                    |
 126          v                  \_________________ | ___________________/
 127                                               \|/
 128                                                v
 129                                          basic.target
 130                                                |
 131           ____________________________________/|                                 emergency.service
 132          /                  |                  |                                         |
 133          |                  |                  |                                         v
 134          v                  v                  v                                 <emphasis>emergency.target</emphasis>
 135      display-        (various system    (various system
 136  manager.service         services           services)
 137          |             required for            |
 138          |            graphical UIs)           v
 139          |                  |           <emphasis>multi-user.target</emphasis>
 140          |                  |                  |
 141          \_________________ | _________________/
 142                            \|/
 143                             v
 144                   <emphasis>graphical.target</emphasis></programlisting>
 145
 146     <para>Target units that are commonly used as boot targets are
 147     <emphasis>emphasized</emphasis>. These units are good choices as
 148     goal targets, for example by passing them to the
 149     <varname>systemd.unit=</varname> kernel command line option (see
 150     <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>)
 151     or by symlinking <filename>default.target</filename> to them.
 152     </para>
 153
 154     <para><filename>timers.target</filename> is pulled-in by
 155     <filename>basic.target</filename> asynchronously. This allows
 156     timers units to depend on services which become only available
 157     later in boot.</para>
 158   </refsect1>
 159
 160   <refsect1>
 161     <title>Bootup in the Initial RAM Disk (initrd)</title>
 162     <para>The initial RAM disk implementation (initrd) can be set up
 163     using systemd as well. In this case, boot up inside the initrd
 164     follows the following structure.</para>
 165
 166     <para>The default target in the initrd is
 167     <filename>initrd.target</filename>. The bootup process begins
 168     identical to the system manager bootup (see above) until it
 169     reaches <filename>basic.target</filename>. From there, systemd
 170     approaches the special target <filename>initrd.target</filename>.
 171
 172     Before any file systems are mounted, it must be determined whether
 173     the system will resume from hibernation or proceed with normal boot.
 174     This is accomplished by <filename>systemd-hibernate-resume@.service</filename>
 175     which must be finished before <filename>local-fs-pre.target</filename>,
 176     so no filesystems can be mounted before the check is complete.
 177
 178     When the root device becomes available,
 179     <filename>initd-root-device.target</filename> is reached.
 180     If the root device can be mounted at
 181     <filename>/sysroot</filename>, the
 182     <filename>sysroot.mount</filename> unit becomes active and
 183     <filename>initrd-root-fs.target</filename> is reached. The service
 184     <filename>initrd-parse-etc.service</filename> scans
 185     <filename>/sysroot/etc/fstab</filename> for a possible
 186     <filename>/usr</filename> mount point and additional entries
 187     marked with the <emphasis>x-initrd.mount</emphasis> option. All
 188     entries found are mounted below <filename>/sysroot</filename>, and
 189     <filename>initrd-fs.target</filename> is reached. The service
 190     <filename>initrd-cleanup.service</filename> isolates to the
 191     <filename>initrd-switch-root.target</filename>, where cleanup
 192     services can run. As the very last step, the
 193     <filename>initrd-switch-root.service</filename> is activated,
 194     which will cause the system to switch its root to
 195     <filename>/sysroot</filename>.
 196     </para>
 197
 198 <programlisting>                                               : (beginning identical to above)
 199                                                :
 200                                                v
 201                                          basic.target
 202                                                |                                 emergency.service
 203                         ______________________/|                                         |
 204                        /                       |                                         v
 205                        |            initrd-root-device.target                    <emphasis>emergency.target</emphasis>
 206                        |                       |
 207                        |                       v
 208                        |                  sysroot.mount
 209                        |                       |
 210                        |                       v
 211                        |             initrd-root-fs.target
 212                        |                       |
 213                        |                       v
 214                        v            initrd-parse-etc.service
 215                 (custom initrd                 |
 216                  services...)                  v
 217                        |            (sysroot-usr.mount and
 218                        |             various mounts marked
 219                        |               with fstab option
 220                        |              x-initrd.mount...)
 221                        |                       |
 222                        |                       v
 223                        |                initrd-fs.target
 224                        \______________________ |
 225                                               \|
 226                                                v
 227                                           initrd.target
 228                                                |
 229                                                v
 230                                      initrd-cleanup.service
 231                                           isolates to
 232                                     initrd-switch-root.target
 233                                                |
 234                                                v
 235                         ______________________/|
 236                        /                       v
 237                        |        initrd-udevadm-cleanup-db.service
 238                        v                       |
 239                 (custom initrd                 |
 240                  services...)                  |
 241                        \______________________ |
 242                                               \|
 243                                                v
 244                                    initrd-switch-root.target
 245                                                |
 246                                                v
 247                                    initrd-switch-root.service
 248                                                |
 249                                                v
 250                                      Transition to Host OS</programlisting>
 251   </refsect1>
 252
 253   <refsect1>
 254     <title>System Manager Shutdown</title>
 255
 256     <para>System shutdown with systemd also consists of various target
 257     units with some minimal ordering structure applied:</para>
 258
 259 <programlisting>                                  (conflicts with  (conflicts with
 260                                     all system     all file system
 261                                      services)     mounts, swaps,
 262                                          |           cryptsetup
 263                                          |          devices, ...)
 264                                          |                |
 265                                          v                v
 266                                   shutdown.target    umount.target
 267                                          |                |
 268                                          \_______   ______/
 269                                                  \ /
 270                                                   v
 271                                          (various low-level
 272                                               services)
 273                                                   |
 274                                                   v
 275                                             final.target
 276                                                   |
 277             _____________________________________/ \_________________________________
 278            /                         |                        |                      \
 279            |                         |                        |                      |
 280            v                         v                        v                      v
 281 systemd-reboot.service   systemd-poweroff.service   systemd-halt.service   systemd-kexec.service
 282            |                         |                        |                      |
 283            v                         v                        v                      v
 284     <emphasis>reboot.target</emphasis>             <emphasis>poweroff.target</emphasis>            <emphasis>halt.target</emphasis>           <emphasis>kexec.target</emphasis></programlisting>
 285
 286     <para>Commonly used system shutdown targets are <emphasis>emphasized</emphasis>.</para>
 287
 288     <para>Note that
 289     <citerefentry><refentrytitle>systemd-halt.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 290     <filename>systemd-reboot.service</filename>, <filename>systemd-poweroff.service</filename> and
 291     <filename>systemd-kexec.service</filename> will transition the system and server manager (PID 1) into the second
 292     phase of system shutdown (implemented in the <filename>systemd-shutdown</filename> binary), which will unmount any
 293     remaining file systems, kill any remaining processes and release any other remaining resources, in a simple and
 294     robust fashion, without taking any service or unit concept into account anymore. At that point, regular
 295     applications and resources are generally terminated and released already, the second phase hence operates only as
 296     safety net for everything that couldn't be stopped or released for some reason during the primary, unit-based
 297     shutdown phase described above.</para>
 298   </refsect1>
 299
 300   <refsect1>
 301     <title>See Also</title>
 302     <para>
 303       <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 304       <citerefentry project='man-pages'><refentrytitle>boot</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
 305       <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
 306       <citerefentry><refentrytitle>systemd.target</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 307       <citerefentry><refentrytitle>systemd-halt.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 308       <citerefentry project='die-net'><refentrytitle>dracut</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 309     </para>
 310   </refsect1>
 311
 312 </refentry>